The zombie shield in Black Ops 2 is a useful tool for protecting yourself against zombie attacks. It has limited durability, and you don’t earn any points, but it can still take a fair amount of punishment. When you aren’t using it, zombie shield protects you from being hit by zombies from behind, and this doesn’t affect its durability.
The Shield only requires two components; a dolly and a car door. Both items can be found in the Diner section of Tranzit; you’ll find the dolly in the bar itself and the car door in the garage. The items move around so I can’t name a more specific location, but you’ll find them in those general places.